Developmental abnormalities in elasmobranchs (sharks and rays) are rarely documented, with reported cases primarily involving fin or cranial deformities. Monozygotic twinning, the formation of identical twins from a single zygote, is particularly rare in these species and has been observed overwhelmingly in viviparous elasmobranchs. Here, we document a rare case of monozygotic twinning in an oviparous shark, the Small-spotted Catshark Scyliorhinus canicula (Linneaus, 1758). This case was characterized by two embryos connected to a single yolk sac via separate yolk stalks within the egg capsule. The embryos exhibited normal development until mortality at developmental Stage 31 (Ballard et al., 1993) or stage 4 (Musa et al., 2018), likely due to stressors such as oxidative stress and allostatic overload, resulting from shared resources within the capsule. This is the first confirmed instance of mortality in monozygotic elasmobranch twins, highlighting the plausible challenges of polyembryony in oviparous elasmobranchs. These findings underscore the importance of understanding reproductive abnormalities and their implications for fecundity, particularly in light of ongoing anthropogenic pressures that threaten elasmobranch populations globally.

This paper describes the first record of the critically endangered gravid female Spiny butterfly ray, Gymnura altavela (Linnaeus, 1758), in the northern Mediterranean and simultaneously the only record of the species in this century for the Adriatic Sea. The female (173 cm disc width and 43.25 kg total weight) was captured at approximately 200 m, off Vlorë in the southern Adriatic Sea off Albania on 12th of May 2022. Upon necropsy, seven near-term fetuses (mean 328 ± 6.47 mm disc width and mean 355 ± 20.06 g in total weight) were transported for further in-vitro incubation. These results represent the deepest capture for the species, as well as the largest litter size and largest size at birth in the Mediterranean Sea which indicates the need for further research.

Angular rough shark, Oxynotus centrina (Linnaeus, 1758), is a poorly known and rare bathydemersal shark inhabiting continental shelves and upper slopes with a significant lack of data and almost no published records in the Adriatic Sea in this century. In this paper, we present 20 new occurrences recorded from May 2015 to September 2021, of which 19 are in Croatian and 1 in Montenegrin territorial waters. Records of juveniles, subadults and adults are reported. The number of described records and available data on HSI/BMI calculations points out that the living conditions are probably most favoured in the area off the Kornati archipelago (central Adriatic Sea), compared to the habitats in the Southern Adriatic where the populations might have significantly lower density. Due to the non-systematic research and non-probabilistic data collection, it is difficult to establish with certainty whether greater number of records in the continental shelf is just an ostensible phenomenon. This article is protected by copyright. All rights reserved.

Melanomacrophages of fish are commonly explored as biomarkers of water pollution and are considered to be sensitive albeit non-specific health indicators in water ecosystems. Sharks as long living marine species are good sentinel species. This study presents morphometric data for splenic and hepatic melanomacrophages (MMC), and observed histopathology in ten lesser-spotted catsharks, Scyliorhinus canicula (L.), one of the most abundant shark species in the eastern Adriatic Sea. At necropsy, we collected random tissue samples from liver, brain, gallblader, pancreas, spleen, kidney, gills, entire digestive system, thyroid gland, rectal gland, entire urogenital (male samples) and genital system (female samples). Collected tissue samples were routinely processed and stained with hematoxylin-eosin, Periodic Acid-Schiff, and Masson Trichrome for microscopic examinations and morphometry. There was a minimal number of histopathological lesions in the examined sharks, but morphometric values reported herein were three folds higher than in previous studies in free-ranging sharks. Studies on larger numbers of sharks are needed to elucidate the biological significance of our finding in the context of population decline of the lesser-spotted catshark.

An initial study of the effects vortex generators (VGs) have on the mitigation of vortex-induced vibrations (VIVs) of hydrofoils have been performed at the Waterpower laboratory of the Norwegian University of Science and Technology. The VGs are placed close to the trailing edge of a blunt hydrofoil. Reasonable design parameters of the VGs were found from the literature with regard to which would induce the strongest vortices with the lowest drag. Vibration frequencies of the hydrofoil were measured using flush mounted strain gauges located close to the trailing edge. This paper presents the design of the VGs and some initial results of the experiments performed with these devices. The preliminary results indicate a possible mitigation of VIVs. This could be because of the interference between the primary vortices generated by the hydrofoil and the longitudinal vortices generated by the VGs, but further work is necessary to make a conclusion.